1. Field of the Invention
The present invention relates to a signal processing method and a signal processing apparatus for processing an acoustic signal obtained by dequantizing a coded acoustic signal and a computer-readable recording medium for causing the signal processing apparatus to function as a computer.
2. Description of the Prior Art
Known as a technique for coding an acoustic signal is MP3 (MPEG 1 Audio Layer 3), AAC (Advanced Audio Coding), ATRAC (Adaptive TRansform Acoustic Coding), WMA (Windows (registered trademark) Media Audio), AC-3 (Audio Code Number 3) and the like. In the MP3 method, for example, an acoustic signal is divided into a plurality of frequency bands and blocked in a unit of varying-length time in order to achieve high efficient compression. The blocked digital data is transformed into a spectrum signal by the MDCT (Modified Discrete Cosine Transform) process and each spectrum 5 signal is further coded by bits which are allocated using the auditory psychology characteristic (see Patent Documents 1 to 3, for example).
An acoustic signal coded as described above is decoded by a decoding apparatus. FIG. 1 is a block diagram showing the lo hardware structure of a conventional decoding apparatus. Denoted at 100 in the figure is a conventional decoding apparatus which comprises an unpacking circuit 101, a dequantizing circuit 102, a frequency-time transforming circuit 103, a frequency band synthesizing circuit 104 and an acoustic signal output unit 105. A 15 coded acoustic signal is inputted into the unpacking circuit 101, and the quantization coefficient, the scale factor, the scale factor multiplexer, the global gain and the subblock gain are respectively unpacked from frame information of the acoustic signal. The coded acoustic signal is then dequantized into an IMDCT (Inverse 20 Modified Discrete Cosine Transform) coefficient by the dequantizing circuit 102 using the quantization coefficient, the scale factor, the scale factor multiplexer, the global gain and the subblock gain.
The IMDCT coefficient obtained by dequantization by the dequantizing circuit 102 undergoes an IMDCT process at the 25 frequency-time transforming circuit 103 for each frequency band and transformed into data in relation to time axis. The inverted frequency band further undergoes band synthesis by an IPFB (Inverse Polyphase Filter Bank), which is a band synthesizing filter, at the frequency band synthesizing circuit 104 and then outputted to the acoustic signal output unit 105 (see Patent Document 3, for example).
Moreover, a technique has been disclosed for complementing a spectrum at the time of decoding with a spectrum for power adjustment in order to compensate for the lack of sense of power caused by compression (see Patent Document 4, for example). In the technique described in Patent Document 4, power adjustment information to be used for complement is generated at a power adjustment information deciding circuit in a coding apparatus based on the characteristic of an input audio signal at the time of coding. Next, the power adjustment information is coded together with the coded audio signal. The coded power adjustment information is then decoded at a power adjustment information decoding circuit in a decoding apparatus and power adjustment information is further generated at a power correction spectrum generating and synthesizing circuit so as to complement the decoded audio signal with the power adjustment information. Moreover, a decoding apparatus for generating, at the time of decoding, expanded frequency spectrum information indicative of the harmonic structure equal to one obtained by expanding the harmonic structure indicated by low frequency spectrum information on the frequency axis to a frequency band which is not expressed by a coded string is also known (see Patent Document 5, for example).
[Patent Document 1]
Japanese Patent Application Laid-Open No. 2002-351500
[Patent Document 2]
Japanese Patent Application Laid-Open No. 2005-195983
[Patent Document 3]
Japanese Patent Application Laid-Open No. 2005-26940
[Patent Document 4]
Japanese Patent Application Laid-Open No. 2003-323198
[Patent Document 5]
Japanese Patent Application Laid-Open No. 2003-108197
However, in the technique wherein an acoustic signal is quantized in the process of coding, there is a problem that rounding or round-down by quantization may cause energy loss of the acoustic signal. Therefore, the user may feel dissatisfaction at the acoustic signal due to the energy loss even at the time of decoding. Moreover, as the technique described in Patent Document 4 is designed to complement power, it is necessary to analyze an input audio signal by a coding apparatus at the time of coding and generate power adjustment information so as to code the signal. Furthermore, it is also necessary to provide a power adjustment information decoding circuit in a decoding apparatus and decode the coded power adjustment information, and there is a problem that interpolation of energy cannot be performed at all for an acoustic signal for which such power adjustment information is not stored. Especially, since coding methods associated with various specifications are made indiscriminately in recent years, there is a problem that the technique described in Patent Document 4 cannot suitably interpolate a coded acoustic signal of various methods. Moreover, as the decoding apparatus described in Patent Document 5 newly generates spectrum information for a frequency band for which a coded string is not expressed, only spectrum information of a low frequency zone is expanded to a high frequency zone and the technique is still not good enough to sufficiently complement dissatisfaction or uncomfortable feeling to an acoustic signal due to an quantization error.